1. Field of the Invention
The present invention relates generally to damping devices of the nature used to support vibrating bodies, and more specifically to damping device which is particularly suited to automotive applications and which features two orifices which are arranged to improve the vibration damping characteristics of the unit.
2. Description of the Prior Art
JP-A-60-104828 describes an example of damping type unit which takes the form of a hollow elastomeric bush member which is filled with an electrorheogical fluid (or ERF as it will be referred to hereinafter) and which is further provided with an electrically controlled orifice unit which divides the device into a working chamber and an expansion chamber. With this device when the bush is compressed, fluid is displaced from the working chamber into the auxiliary one (defined between the orifice unit and a relative flexible diaphragm) and vice versa. By applying a high voltage to across the electrodes of the orifice, the viscosity of the ERF can be induced to undergo a remarkable increase and the fluid induced to assume to an almost solid state. Under these conditions the orifice is either effectively blocked or the restrictive properties thereof remarkably increased. When this type of arrangement is used to support automotive engines for example, it is possible to improve the effective vibration damping characteristics of the system defined by the engine, mounts and vehicle chassis particularly at low frequencies. For example, when the engine is idling or undergoes what shall be referred to as engine shake, the vibrations produced exhibit low frequency large amplitude characteristics.
However, this type of arrangement has suffered from the drawback that even though the above vibration attenuation tends to be very effective in the vibration frequency range in which idle and shake and vibration occur, the device has proven somewhat ineffective in that the spring constant of the device tends to be raised excessively in that the vibration frequency range in which reverberation noise is produced in the vehicle cabin.
In this specification it will be assumed for the sake of discussion that the engine "shake" vibrations exhibit a frequency in the order of 10 Hz while the engine "idling" vibrations exhibit a frequency in the order of 25 Hz. On the other hand, the vibrational range in which the reverberation tends to occur line in the 80-100 Hz region.